1. Field of the invention
The present invention relates to a method for producing rolled structural shapes used in vehicle door frames and the like.
2. Description of the Related Art
Japanese Utility model laid-open No. 6 (1994)-32139 discloses a hemmed roll formed shape used as a pillar member of a vehicle door frame comprising a first support recess, a second support recess which opens opposite from the opening of the first support recess, and a flat exterior section that extends generally in the opening direction of the first and second support recesses, and that has a side edge with a gradually changing cross-sectional shape. In making this conventional product, a metal sheet is formed by a roll forming mill, for example, into an intermediate product having the first and second support recesses and a flat exterior portion, which initially, does not have the gradually changed sectional shape. In this intermediate product, the flat exterior portion is defined by layered portions of the metal sheet, and the two opposite side edges of the metal sheet are arranged stepwise at one of the sides of the flat exterior portion. Then, one of the side edge portions is cut off by a press machine, for example, to provide the flat exterior portion with desired edge configuration having the gradually changed sectional shape. The other of the side edge portions is then bent and folded along the one side edge by a roll forming mill, for example, so that the bent edge portion encloses the one edge, thus forming a side hem.
However, in this conventional method, the cutting blade of the press machine must be operated to provide edges having the gradually changing sectional shape while the workpiece is being moved at, or substantially at the sheet conveyance speed. Therefore, the method requires a mechanism for sliding the press machine along the production line and a space for the sliding of the press machine laterally, thus increasing the size of the production apparatus. This makes it difficult to conduct the step of providing edges having the gradually changing sectional shape and the roll forming step in a single continuous production line. In addition, the method requires that the side edges of a metal sheet be generally aligned at one side end of the flat exterior portion in order to form an edge having the gradually changed sectional shape. This requirement limits the variety of sectional shapes of vehicle door frames that can be produced, that is, it may be very difficult to produce an ideal sectional shape. Furthermore, since only one of the side edge portions of a metal sheet is cut by a press machine or the like, the hemming step may form a gap in the hem, more specifically, between the bend of the outer layer portion and the edge of the inner layer portion. The design quality of the flat exterior portion may be degraded. For example, parts of the flat design portion may become undesirably contoured.